U.S. Pat. No. 4,016,907 (Apr. 12, 1977) discloses a commonly used railway tank car lading valve assembly.
However, when installed on a railway tank car, the combined distance of the mounting flange and the valve housing extends downwardly below the lower external surface of the tank bottom in excess of one (1) inch. A skid is therefore required in accordance with AAR specifications for tank cars (Specification M-1002) paragraph E10.00, to protect the valve assembly against shearing off on impact. These regulations require the skid have a three (3) unit longitudinal taper for each single unit of projection in excess of one (1) inch below the external tank bottom surface, and requires that this taper having a sliding surface without discontinuities. This skid adds considerable weight to the car, and adds expenses of material and labor to install the skid.
A butterfly valve is moved between open and closed positions by a shaft which extends through the valve housing on one side of the outlet. This portion of the valve housing adjacent to the shaft does not touch the mounting flange and this creates a gap which cannot carry impact loads having significant components applied vertically, upwardly into the tank mounting flange. In addition, U.S. Pat. Nos. 4,180,242; 4,158,453; 4,184,663; 4,234,158 and 4,234,159 disclose combination skids and valve seats which are tapered in the longitudinal direction to comply with the above AAR regulations. U.S. Pat. No. 4,212,447 discloses a tapered tank car mounting flange. U.S. Pat. No. 4,237,928 discloses a low profile tapered sump.